The present invention relates to positive electrodes for use in electrochemical energy storage devices. More particularly, the invention relates to lithium insertion positive electrodes containing either lithium nickel cobalt oxides or mixtures of such materials with other compounds.
Due to the increasing demand for battery-powered electronic equipment, there has been a corresponding increase in demand for rechargeable battery cells having high specific energies. In order to meet this demand, various types of rechargeable cells have been developed, including improved aqueous nickel-cadmium batteries, various formulations of aqueous nickel-metal hydride batteries and, recently, nonaqueous rechargeable lithium-ion cells (sometimes referred to as "lithium rocking chair," or "lithium intercalation" cells). Lithium-ion cells are particularly attractive because they have a high cell voltage and a high specific energy.
Various positive electrodes ("cathodes" on discharge) have been studied and/or used in lithium ion batteries. These include lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides. The preparation and use of lithium transition metal oxide positive electrodes are described in various publications including U.S. Pat. Nos. 4,302,518 and 4,357,215 issued to Goodenough et al., which are incorporated herein by reference for all purposes.
While these materials, particularly lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2), and lithium manganese oxide spinnel (LiMn.sub.2 O.sub.4), have been found somewhat adequate, they each have some serious shortcomings. For example, they may have an unacceptably high irreversible capacity loss. This loss occurs during a first charge cycle when the cell's negative electrode undergoes formation.
"Formation" refers to electrode modification processes employed after a cell is assembled, but before it is reversibly cycled; note that some but not all cell types require formation. In cells that require it, formation electrochemically modifies the cell's electrodes so that thereafter they can be reversibly cycled. In lithium ion cells, formation involves an initial cycle which irreversibly drives some lithium ions from the positive electrode material to a carbon negative electrode ("anode" on discharge) where they are believed to form a surface layer that has been found necessary to provide high energy cycling. This surface layer is known as a solid electrolyte interface or "SEI."
The ratio of the first cycle charge capacity over the first cycle discharge capacity for a positive electrode is an important parameter in lithium ion cell design. This ratio should be compared to the same ratio for the cell's negative electrode. In all cases, the positive electrode's first cycle capacity ratio should be designed to match the negative electrode's first cycle capacity ratio. If the positive electrode ratio exceeds the negative electrode ratio, lithium metal electroplating can occur, which can result in undesirable capacity fading and safety problems. In the case where this ratio is larger for the negative electrode, the cell's reversible capacity is limited by the negative electrode. Likewise, when the opposite is true, the cell is limited by the positive electrode.
The problem of a positive electrode with a high first cycle ratio can be further understood by considering the example of lithium nickel oxide. As this material has a high first cycle charge ratio, less of it is required to "form" a given amount of carbon negative electrode than is required to reversibly cycle against that same amount of negative electrode (assuming that the negative electrode has a lower first charge ratio). Thus, if a cell is provided with an amount of lithium nickel oxide sufficient for formation, that cell will have insufficient lithium nickel oxide to utilize the available negative electrode material during subsequent reversible cycles. That is, the negative electrode will be underutilized, with some fraction of it constituting useless mass (which reduces the cell's specific energy). On the other hand, if more lithium nickel oxide is used in the cell (beyond that required for formation), some metallic lithium will electroplate onto the negative electrode during formation, presenting the danger that the electroplated lithium metal will undergo an exothermic chemical reaction.
By designing a mixed oxide to include nickel plus another metal which tends to equalize the amount of oxide required to reversibly cycle against and form a given amount of negative electrode material, the above difficulties can be mitigated. Lithium nickel cobalt oxides are potentially useful candidates for such applications because the presence of cobalt does, in fact, tend to equalize the amount of oxide required for these two functions. Note that, in contrast to lithium nickel oxide, more lithium cobalt oxide is required to form a negative electrode than to reversibly cycle against it. Thus, it intuitively follows that the presence of cobalt in a nickel oxide will tend to match the formation capacity and reversible capacity of the oxide.
The presence of cobalt has another advantage. During reversible cell cycling, it reduces the average oxidation state of transition metals in the oxide lattice. A fresh uncycled positive electrode could have the formula LiMO.sub.2, with the valence of M being equal to 3. On full charge, the positive electrode oxide could in theory have a formula MO.sub.2, with the valence of M being equal to 4. Thus, during charge the transition metal's oxidation state increases, and during discharge the transition metal's oxidation state decreases. Because some lithium in the positive electrode is irreversibly lost during the formation cycle, the positive electrode matrix can never discharge the whole way to its initial stoichiometry of LiMO.sub.2. As a consequence, the valence of M is never lowered all the way to 3 (during normal reversible cycling). Rather, the oxidation state of M is bracketed between a value greater than 3 and a value lower than 4 during reversible cycling. The bounds of this oxidation state are determined by how much lithium is lost during formation.
Obviously, compounds with high irreversible capacity losses will cycle at higher average oxidation states of M than compounds with lower irreversible capacity losses. Lithium nickel oxide has a much higher irreversible capacity loss (about 40 mA.multidot.hr/gm) than lithium cobalt oxide (about 8 mA.multidot.hr/gm). The introduction of cobalt into the LiNiO.sub.2 matrix reduces the irreversible capacity loss and thereby reduces the oxidation state of M during reversible cycling. As a consequence, the oxide lattice is at a lower oxidation state and therefore less reactive and less likely to pose a significant safety risk.
Nevertheless, LiNi.sub.a Co.sub.b O.sub.2 compounds still pose the risk of decomposing on severe overcharge. Such decomposition reaction is accompanied by a release of oxygen and energy which increases the cell's internal temperature and pressure, and thereby increases the risk of igniting the electrolyte. Obviously, designs that avoid these potential problems will be of significant commercial importance.
Note that the lithium cobalt oxide and lithium nickel oxide also may undergo a decomposition reaction on overcharge. Generally, it is known that LiCoO.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4 have varying degrees of thermal stability in their delithiated forms (see, e.g., J. R. Dahn et al., Solid State Ionics 69 (1994) 265-270). For example, it is known that the layered compound LiNi.sub.0.5 O.sub.2 is transformed to the spinnel LiNi.sub.2 O.sub.4 on heating to above 200.degree. C. This transformation is accompanied by little mass loss or heat generation. In contrast, at higher degrees of delithiation (e.g., Li.sub.0.3 NiO.sub.2), the transformation to spinnel is also accompanied by significant oxygen generation and heat liberation. Delithiated LiCoO.sub.2 does not undergo a transformation to spinner form, but rather decomposes to layered LiCoO.sub.2 and stable Co.sub.3 O.sub.4 at about 245.degree. C. Some oxygen is also released in this reaction.
Still other problems remain in many lithium metal oxide positive electrodes. For example, many lithium metal oxides (e.g., LiMn.sub.2 O.sub.4, LiCoO.sub.2, LiNiO.sub.2, and some atomic mixtures of Mn, Co, and Ni oxides) have substantially flat discharge profiles. That is, their voltage varies only slightly with state of charge until very nearly all of their capacity has been exhausted. Thus, from full charge until nearly complete discharge (during which time most available lithium enters the positive electrode), the electrode voltage remains high and nearly constant. Only when most available lithium has been extracted from positive electrode (at the end of discharge) does it exhibit a characteristic sharp drop in voltage. While such discharge characteristics provide high and relatively constant potentials during most of discharge, they can cause cells to perform poorly at high rates of discharge. This results because ohmically caused variations of potential within the electrode can not be compensated by variations in the reaction rate. Thus, the electrode material is under-utilized at any given state of discharge, thereby limiting the rate, energy, and cycling performance of cells, as discussed in T. Fuller et al. J. Electrochem. Soc., 1, 114 (1994), incorporated herein by reference for all purposes.
In view of the above, there is a need for improved lithium insertion positive electrode materials which have substantially matched formation and reversible cycling capacities, resist decomposition on overcharge, and have sloping discharge profiles.